At the High Court in Livingston, the judge Lord Boyd noted that Bradley "was a man of violence" who had 10 convictions for assault.

He told Grant: "But you, of course you did not know that. What you did know was that earlier that night Patrick Bradley had robbed your son of his mobile phone at knifepoint.

"Had he been caught it seems likely, given his record, that he would have been prosecuted in this court, the High Court, and on conviction would have received a High Court sentence.

"As it happens you had the means to bring him to justice because the phone had been easily located by the Find My iPhone app."

The judge added that all he needed to do was to phone the police and give them the information, but instead he took a knife from his home and went with his three sons to look for the robber.

He said Grant's lawyer had earlier described that decision as an error of judgment, but added that he could not accept the description, saying: “It's clear you were prepared to be met with violence, or at least the threat of violence, and went armed to meet it."

Lord Boyd acknowledged that Grant had lost an eye and now had a lifelong disability which had effectively ended his career as a delivery driver.

He described his conviction as a "tragedy" for his family and told him the proud educational record of his sons was a credit to him.

The judge added: "I take into account you will have to live with the physical consequences of that event for the rest of your life and I have reduced the sentence accordingly.

"Nevertheless, the fact is you took the law into your own hands and went looking for a man you knew was armed with a knife, armed with a weapon of your own."

A previous hearing was told that Jordan Grant had been returning to his home in Greenock when he was confronted by Bradley. He ordered the student to hand over his iPhone, which he did.

He then went home and told his father, who used the mobile app to trace the stolen phone to a nearby location. Grant, armed with a knife, then left his home with Jordan and his two younger sons, Lee, 17, and Jamie, 16, to find Bradley.

When he ordered him to hand over the phone, Bradley lashed out at him, stabbing him in the eye, before Grant stabbed him five times with the knife he had picked up before leaving home. Bradley suffered a cardiac arrest after being stabbed in the chest and died in hospital.

Grant's three sons originally faced murder charges but their not guilty pleas were accepted.